German Pat. No. 637,124 shows a mechanical device for blocking carriage movement when the machine is out of paper. To this end a two-arm lever is used. One arm is moved when the paper is introduced causing the other arm to swing out from blocking relation from the carriage so that it can move freely. When the paper is at the end, the second arm swings back into blocking relation with the carriage. In this device the paper edge which must operate the lever can be easily damaged. Besides, this device cannot recognize a failure of the paper to move.
German Pat. No. 2,258,546, corresponding to U.S. Pat. No. 3,949,856, discloses a device for monitoring feed movement of endless paper webs adapted to be driven by pins engageable with holes in the margin. Two rotating parts are used to generate reference signals which are compared with each other. When the comparison exceeds or drops below a certain limit value, the machine shuts itself off. This device is elaborate and expensive, as it requires two pulse generators.
Swiss Pat. No. 568,226 discloses a pulse generator driven by paper movement. This device is only suitable for monitoring long material webs, but not for individual paper sheets.
In accordance with the invention a pulse generator generates pulse sequences only in response to rotation of a drive roll by paper advanced by a line indexable platen. If there is no paper inserted or if paper does not move incident to platen indexing movement, no pulse sequences occur at the output of the pulse generator and this condition stops the machine.
An object of the invention is to provide a low-cost device for monitoring paper feed movement of endless and individual papers and which can also recognize whether the paper is in the printing range at all.
Another object of the invention is in the provision of a simple, easily-implemented paper monitoring device which is designed to avoid paper damage.